The present invention relates generally to the field of polarization diversity based optical devices.
Many optical devices, such as diffractometers, spectral analyzers, or configurable add/drop demultiplexers, disperse a beam of light using a diffraction grating. Most devices include a collimator to make all the light incident on the grating or a prism parallel. Collimating is necessary to make sure that all light is incident at the same angle.
The efficiency of a grating is dependent on the polarization of the incident light. Highly dispersive diffraction gratings, such as 1200 l/mm reflection gratings or 1800 l/mm transmission gratings for 1.55 xcexcm applications, suffer from a high Polarization Dependent Loss (PDL). In order to use these components, the design of such devices needs to incorporate a polarization balancing or polarization diversity.
Polarization balancing means that a double pass is forced on the diffraction grating with orthogonal polarization states. This averages the polarization sensitivity of the gratings. However, as a result, a large loss penalty in the order of 2.5 dB or more is encountered for highly dispersive gratings.
U.S. Pat. No. 5,886,785 discloses an optical spectrum analyzer wherein polarization balancing is used. A grating assembly using a Littmann-Metcalf configuration includes a reflector that forces a double pass on the grating. This assembly gives good optical stability by self-alignment when the reflector is a dihedron and gives a very good angular dispersion, which improves the optical resolution.
In the Littmann-Metcalf configuration, several parameters may be used to improve the resolution, but each of them causes specific problems. Thus, the improvement obtained by adopting a low grating pitch or groove spacing p causes a problem in the size of the system. Furthermore, since the analyzer includes a collimating lens of focal length f, increasing the focal length f increases the dispersion in the focal plane which improves the resolution. However, the dependence on chromatism is then increased, which causes difficulty in collimation and refocusing for some wavelengths. The resolution can also be improved by adopting a grazing incidence on the grating, but this causes an efficiency drop.
Furthermore, the analyzer in the Littmann-Metcalf configuration is sensitive to polarization of the incident wave, since the grating efficiency is very dependent on polarization for low angles of incidence on the grating. This causes a significant variation in the signal level with polarization.
A polarization diversity circuit, on the other hand, splits the incoming light into two sub-beams of orthogonal polarization by using a polarization beam splitter, such as a birefringent crystal. One of the two sub-beams is rotated by a halfwave plate so that the polarization states of both sub-beams are parallel. In this manner, one of the sub-beams is rotated to a preferred polarization state of the diffraction grating to reduce the loss penalty to less than 0.7 dB. Hence, the use of a polarization diversity is advantageous with highly dispersive diffraction gratings.
The common techniques to implement polarization diversity, such as using thermally expanded core (TEC) fibers or micro-collimated beams, narrow the numerical aperture (NA) of the incoming beam of light before splitting the incoming beam into two orthogonal polarization states in a birefringent crystal to achieve non-overlapping beams. However, these techniques affect the resolution of the grating and hence affect the resolution of any optical device based on diffraction gratings, such as demultiplexers, configurable optical add/drop demultiplexers (COADM), dynamic gain equalizers (DGE), or optical spectrum analyzers (OSA), because the beam size is smaller on the grating, illuminating a smaller number of lines.
It is an object of this invention to provide a polarization diversity circuit having a low loss penalty.
It is another object of the invention to maximize the beam size on the grating for improving the resolution of the grating.
Thus, it is an object of this invention to provide a polarization diversity circuit yielding high resolution optical devices, particularly optical devices that are based on diffraction gratings.
In accordance with the invention there is provided, a polarization diversity circuit comprising a cylindrical lens for receiving an input beam and for providing an output beam having a substantially eccentric cross-section; and a birefringent crystal optically coupled with the lens so as to receive the output beam from the lens and to provide two eccentric orthogonally polarized sub-beams each having a substantially eccentric cross-section.
In accordance with the invention, there is further provided a polarization diversity circuit comprising: a cylindrical optical element for receiving an input beam of light and for providing an output beam having a substantially elliptical cross-section, said elliptical cross-section having a major axis and a minor axis, said output beam being substantially collimated in a direction of the minor axis and substantially diverging in a direction of the major axis; and a birefringent element for receiving the output beam from the cylindrical optical element and for separating the output beam into two sub-beams having substantially orthogonal polarization states and having substantially elliptical cross-sections, said sub-beams being substantially collimated in the direction of the minor axis and substantially diverging in the direction of the major axis, and wherein the birefringent element is arranged to cause a displacement of one of the sub-beams in the direction of the minor axis.
In accordance with an embodiment of the invention the cylindrical optical element is a cylindrical lens. In accordance with a further embodiment of the invention the cylindrical optical element is a cylindrical mirror.
In accordance with another aspect of the invention, there is provided, a method of providing two spatially separated orthogonally polarized beams of light, comprising the steps of launching an input beam into and through an optical element having optical power that will provide an output beam having an eccentric cross-section; and launching the output beam into a birefringent crystal to obtain two orthogonally polarized spatially non-overlapping sub-beams.